1. Field of the Invention
The present invention relates to a method of manufacturing toner for use in electrophotography and a toner manufactured by the method.
2. Discussion of the Background
A series of electrophotographic image forming processes include, in rough order, a charging process, an irradiating process, a developing process, a transfer process, a cleaning process, and a fixing process. In the charging process, a surface of a photoreceptor is evenly charged. In the irradiating process, the charged surface of the photoreceptor is irradiated with a light beam to form an electrostatic latent image thereon. In the developing process, a developer such as a toner is adhered to the electrostatic latent image formed on the surface of the photoreceptor to form a toner image. In the transfer process, the toner image transferred onto a recording medium is fixed thereon by application of heat and/or pressure. In the cleaning process, residual toner particles that remain on the surface of the photoreceptor without being transferred onto the recording medium are removed. Thus, a resultant image is formed on the recording medium.
The toner used for the developing process typically includes a binder resin, a colorant such as carbon black, and a release agent such as wax. The binder resin has a function of binding the colorant and the release agent that cannot be formed into a toner alone or fixed on a recording medium by themselves. The colorant is necessary for forming a full-color image, and organic pigments and dyes are generally used therefor. The release agent functions as an offset inhibitor when a fixing member such as a heat roller contacts a toner image which is transferred onto a recording medium in the fixing process.
In accordance with recent speed-up of electrophotographic image forming processes, in other words, shortening of a processing time for recording an image on a sheet of paper, a time of contact of a fixing member with the paper is also shortened. As a consequence, a temperature range of the fixing member within which an offset problem does not occur (hereinafter referred to as a non-offset temperature range) is narrowed. The offset problem here refers to an undesirable phenomenon in which part of a fused toner image is adhered to a surface of the fixing member, and re-transferred onto an undesired portion of the recording medium. Accordingly, toners are required to have improved releasability from the recording paper to prevent narrowing of the non-offset temperature range, and one proposed approach involves increasing the content of a release agent in a toner. It is to be noted that a release agent having a narrow molecular weight distribution and a low meting point tends to be unevenly dispersed in a toner, and easily exposes at a surface of the toner.
In contemporary cleaning processes, residual toner particles that remain on a surface of a photoreceptor without being transferred onto a recording medium are collected so that the collected residual toner particles (hereinafter referred to as recycled toner particles) are reused in the developing process. However, the recycled toner particles may be excessively exposed to heat and pressure when transported through a collection path. As a result, a release agent may be exposed at a surface of the recycled toner particle, particularly when the release agent is unevenly dispersed therein. Such a release agent exposed at the surface of the recycled toner particle may form an undesirable film on a photoreceptor or a surface of a carrier, shortening lives thereof. Accordingly, toners have been developed to have both improved releasability and durability.
The colorant is required to be evenly dispersed in the toner. If the colorant is unevenly dispersed therein, not only the resultant image density but also the electric resistance of the toner may be too low. Such a toner may not be efficiently transferred in the transfer process, and as a result, the amount of the recycled toner particles that are excessively exposed to heat and pressure at a collection path increases, resulting in deterioration of the developer.
Methods of manufacturing toner to meet the requirements described above are broadly classified into pulverization methods and polymerization methods. From the viewpoint of cost and ease of color change in continuous manufacturing, pulverization methods are preferable.
A typical pulverization method includes processes of mixing raw materials including a binder resin, a colorant, and a release agent; melt-kneading and cooling the raw material mixture; coarsely and finely pulverizing the kneaded raw material mixture into fine particles; classifying the pulverized fine particles; and adding (adhering) an external additive to the surfaces of the classified fine particles. The dispersion state of the release agent in the resultant toner is controlled in the mixing process. The melt-kneading process is typically performed using an open system roll type kneader, an extruder including a barrel including a screw, or the like. Open system roll type kneaders are widely used recently because the kneading can be performed at a relatively low temperature and a relatively high viscosity.
For example, Japanese Patent No. (hereinafter “JP”) 3366576 discloses a method of manufacturing toner using an open system roll type kneader. It is disclosed therein that the open system roll type kneader contributes to improvement of the dispersion state of a wax in the resultant toner, because such an open system roll type kneader is capable of kneading a raw material mixture by applying a high shearing force at low temperatures.
However, there is a possibility of degrading thermal properties of a binder resin when trying to finely disperse both the wax and a colorant therein, because the colorant needs a high shearing force, whereas the release agent needs a low shearing force, to be finely dispersed in the resultant toner. When a high shearing force is applied to the binder resin, molecular chains thereof may be cut, resulting in deterioration of elasticity of the binder resin. Consequently, the resultant toner may easily cause the above-described offset problem at high temperatures, narrowing the non-offset temperature range.
In addition, the release agent may be excessively dispersed and formed into too small particles in the resultant toner when such a high shearing force is applied thereto. Consequently, the resultant toner may have insufficient releasability from a fixing member, narrowing the non-offset temperature range.
Various studies have been made of the mixing process in which raw materials of a toner are mixed.
For example, Unexamined Japanese Patent Application Publication No. (hereinafter “JP-A”) 2004-317872 discloses a method of manufacturing toner in which a process for mixing raw materials including a binder resin and a charge control agent is performed under a specific mixing condition.
As another example, JP-A 2005-234370 discloses a method of manufacturing toner in which a process for mixing a charge control agent with a binder resin includes a plurality of steps, so that the charge control agent is finely dispersed in the resultant toner.
Both JP-A 2004-317872 and JP-A 2005-234370 improve the dispersion state of charge control agent in the resultant toner. However, neither of them improves those of release agent and colorant. Accordingly, neither JP-A 2004-317872 nor JP-A 2005-234370 may contribute to improvement of transferability and fixability of the resultant toner.
JP2993624, JP3010326, JP3603709, JP3801458, JP3861978, JP-A 2004-341129, JP-A 2005-345733, JP-A 2006-305982 and JP-A 2007-003799 each disclose a method of manufacturing toner including a first mixing process and a second mixing process, at least one of which is performed using an open system roll type kneader.
These methods have an advantage in that raw materials such as a release agent and a colorant can be finely dispersed in the resultant toner. However, at the same time, molecular chains of a binder resin may be cut in the double mixing processes, resulting in a narrow non-offset temperature range. In a case in which the dispersion state of colorant favored over that of release agent, the release agent may be excessively dispersed and formed into too small particles in the resultant toner. As a consequence, the resultant toner may have insufficient releasability from a fixing member, narrowing the non-offset temperature range. By contrast, in a case in which the dispersion state of release agent favored over that of colorant, the colorant may be insufficiently dispersed in the resultant toner, causing deterioration of the resultant image density and transferability.